The present invention relates to an arrangement of mounting a chip on a substrate, and more particularly to an interposer device for use in an arrangement of mounting a chip on a substrate for operatively connecting chip electrodes with substrate pads in a liquid-crystal display (LCD).
Liquid-crystal display (LCD) technology is commonly used in mobile telephone portable PCs and other portable electronic devices. Increasing demands for displays providing even higher reformation content. e.g greyscale functionality, larger number of pixels, etc, result in an increased silicon area of the LCD driver chip, However, the area of the driver chip already constitutes a significant part of the total silicon area in for example state of the art mobile phones.
A Chip on Glass (COG) LCD is an LCD having a semiconductor or an LCD driver chip mounted directly on an LCD substrate, preferably a glass substrate. COG has become commonly used due to its relatively low cost and that the mounting method is space efficient and provides a compact solution. However, conventional COG has a disadvantage. The pad layouts of an LCD driver used in the conventional COG implementation have to meet certain restrictions. The restrictions are mainly due to the layouts rules that the conductive line pattern on the LCD substrate has to follow.
U.S. Pat. No. 5,467,210 discloses an arrangement of bonding an IC chip to an LCD device. The IC chip is a xe2x80x9cbare IC chipxe2x80x9d in which a single surface of a semiconductor bulk has multiple semiconductor devices fabricated thereon and a passivation film covers the semiconductor devices fabricated side, so that only terminals for connection to a device outside the chip are uncovered. The bare IC chip lacks a lead frame to which the bare IC chip is bonded, leads, bonding wires connecting the terminals of the bare IC chip to the leads, and a package of ceramic etc.
A prior art solution of a Chip On Glass LCD device according to U.S. Pat. No. 5,467,210 or the like is illustrated in FIGS. 1 and 2. According to FIG. 1, electrodes 1 are provided on a single line along the circumference of a driver chip 2, which is the only possible pattern for the electrodes. Segment output electrodes are provided along one side 3, and row output electrodes are split up in two halves on each side 4 and 4xe2x80x2 of the segment output electrodes 3. Further, the logic supply input electrodes and the I/O interface electrodes are placed on the opposite side 5 of the chip to the segment output electrodes 3, The chip 2 is mounted on a lower glass substrate 6 in the form of a rectangular board by a bonding method. In the bonding method employed in U.S. Pat. No. 5,467,210, the chip includes bumps swelling from the electrodes. An anisotropic electrically conductive adhesive is applied to substrate pads 7 on the substrate 6. The anisotropic electrically conductive adhesive comprises an adhesive medium and electrically conductive particles dispersed in the adhesive medium so as to separate from each other. The anisotropic electrically conductive adhesive possesses an electrical conductivity only along the thickness thereof. The chip 2 is bonded to the pads 7 of the lower glass substrate 6 so that the bumps of the electrodes 1 are bonded to the pads 7.
A top view of an LCD device comprising the chip 2 mounted on the lower glass substrate 6 as described above is shown in FIG. 2. Further, an upper glass substrate 8 in the form of a rectangular board is mounted on the larger lower glass substrate 6, and a rectangular area 9 including a grid constitutes a display area in which data lines and scanning lines cross in the form of a matrix.
An obvious solution in order to limit the increase of the silicon area and/or reduce the total silicon area of the driver chip 2 would be to use silicon processes with finer line widths.
Today, a minimum pad pitch of about 50-100 xcexcm can be used on the substrate whilst maintaining an acceptable yield loss from the driver mounting process. An LCD driver chip has several hundred electrodes for connection to the pads of the glass substrate, which sets a minimum silicon area of the chip. If the silicon area needed for the implementation of the functionality of the driver is less than the area set by the circumference on the chip needed to lay out the pads with the associated pad pitch, the driver is pad-limited. A further reduction of the chip area needed for the functionality by using silicon processes with finer line widths will, however, not lead to a smaller total chip area. Increasing demands for displays providing even higher information content result in driver chips with an increased number of pads and, consequently, a larger chip size. As described above, the driver chip will be pad-limited and the total chip area will not decrease even if smaller line widths are used.
It is therefore an object of the present invention to provide an improved arrangement of mounting an IC chip on a substrate in order to reduce the problem of pad limitation.
This object is accomplished by an interposer device according to the invention for use in the arrangement of mounting the IC chip on a substrate. interposer electrodes are mounted on said interposer device according to a first pattern corresponding to a configuration of chip electrodes on the IC chip with a reduced electrode pitch. Further, interposer pads are mounted on said interposer device according to a second pattern corresponding to a configuration of substrate pads with a particular pad pitch on the substrate. In order to operatively connecting the chip electrodes with the corresponding substrate pads, the interposer pads are interconnected with interposer electrodes via interconnecting leads printed on the interposer.
Another object of the invention is to provide an improved liquid-crystal display (LCD) with a reduced silicon area on the LCD driver, whereby the disadvantages caused by substrate pad-limitation are reduced.
This object is accomplished by a liquid-crystal display (LCD) according to the invention, including an interposer device mounted between the IC chip and the substrate, wherein said interposer device operatively interconnects substrate pads configured in a first pattern on the substrate with electrodes configured in a second pattern on the driver chip, wherein said electrodes have a reduced electrode pitch essentially smaller than the pad pitch of the substrate pads.
Still another object of the invention is to provide an improved method of mounting an IC chip on a substrate in order to reduce the problem of pad limitation.
This object of the invention is accomplished by a method according to the invention, wherein the chip is bonded to an interposer device, and then the interposer device is bonded to the substrate, wherein the interposer device interconnects substrate pads configured in a first pattern on the substrate with electrodes configured in a second pattern, wherein said electrodes have a reduced electrode pitch essentially smaller than the pad pitch of the substrate pads.
An advantage of the LCD according to the invention is that a total cost reduction is achieved, because the cost per areas of the interposer means is lesser than the cost per area of the chip.